Multi-branched paraffins are ideal gasoline-blending components possessing high octane numbers. For environmental reasons, there is a need to find substitutes for aromatic components in gasoline. Therefore, there is an incentive to develop a process for increasing the octane number of the C4-C12 cuts. While C5/C6 paraffin isomerization is a common refinery process, commercialisation of processes including higher fractions (C7+ hydrocarbons) meets significant difficulties given by the usually high degree of cracking to gaseous products, which is undesirable.
An article by K. Arata and M. Hino in Proceedings 9th International Congress on Catalysis (1988, vol. 4, pages 1727-1735) describes a catalyst based on a Group IVB metal oxide such as zirconia in particular modified by an oxyanion of the Group VIB particularly tungstate and its use in paraffin isomerization.
Further research activities have shown that the catalytic performance of tungstated zirconia catalysts in paraffin isomerization may be improved by addition of a hydrogenation/dehydrogenation function, preferentially a noble metal, to the mixed solid acid catalysts. The use of tungstated zirconia promoted with noble metal in paraffin isomerization has been described in the open literature for instance by S. L. Soled, S. Miseo, J. E. Baumgartner, W. E. Gates, D. G. Barton and E. Iglesia, Proc. 13th Int. Conf. Catal. (The Taniguchi Foundation, Kobe, Japan, 1994) page 17; E. Iglesia, D. G. Barton, S. L. Soled, S. Miseo, J. E. Baumgartner, W. E. Gates, G. A. Fuentes and G. D. Meitzner, Stud. Surf. Sci. Catal. 101 (1996) 533; G. Larsen, E. Lotero, S. Raghavan, R.D. Parra and C.A. Querini, Appl. Catal. A 139 (1996) 201.
A series of U.S. patents on solid acid isomerization catalysts have been assigned to Mobil Oil Corporation. U.S. Pat. No. 5,510,309 provides a method for preparing an acidic solid comprising a Group IVB metal oxide modified with an oxyanion of a Group VIB metal. An example of this acidic solid is zirconia modified with tungstate. This modified solid oxide may be used as a catalyst for example to isomerise C4 to C8 paraffins. The modified solid oxide is prepared by co-precipitating the Group IVB metal oxide along with the oxyanion of the Group VIB metal. After filtration, the co-precipitate is calcined at 825° C. U.S. Pat. No. 5,780,382 provides a method for preparing an acidic solid comprising a Group IVB metal oxide modified with an oxyanion of a Group VIB metal.
U.S. Pat. No. 5,854,170 describes the preparation of a noble metal containing an acidic solid catalyst by impregnation of a Group IVB metal hydroxide or hydrated oxide with an aqueous solution comprising an oxyanion of a Group VIB metal. The noble metal (preferentially Pt) may be added by co-impregnation with the oxyanion of the Group VIB metal or in a separate impregnation step.
U.S. Pat. No. 6,080,904 describes a C4-C8 isomerization process utilising an isomerization catalyst with a hydrogenation/dehydrogenation component (preferentially Pt) and with solid acid component comprising a Group IVB metal oxide modified with an oxyanion of a Group VIB metal.
In all the above-mentioned patents, aluminium is mentioned merely as a conventional matrix material such as alumina, silica-alumina and silica with preference given to silica.
The tungstated zirconia system has frequently been described as catalyst for C5+ isomerization. The catalysts typically contain tungsten oxide in a concentration below 20 wt % and hydrogenation component is platinum. However, the catalytic selectivity towards isomerization delivered by these materials is not sufficient. The following patents are variations of the above-mentioned prior art.
U.S. Pat. No. 5,422,327 describes a catalyst composition of a Group VIII metal incorporated into a support consisting of zirconia, said support being further impregnated with a mixture of silica and tungsten oxide and its use in paraffin isomerization.
U.S. Pat. No. 5,648,589 claims a catalytic isomerization process comprising contacting a C5+ feed under isomerization conditions with a catalyst composition consisting of a Group VIII metal and a zirconia support impregnated with tungsten oxide and silica.
U.S. Pat. No. 5,837,641 describes an isomerization reaction over tungstated zirconia and the promotional effect of water on this catalyst.
U.S. Pat. No. 6,767,859 describes an alkane isomerization process using a catalyst composition of a metallic oxide doped by a metal dopant, a Group IVB metal and a hydrogenation/dehydrogenation function, the metal-doped metallic oxide being prepared by co-precipitation from solution and the metal dopant being incorporated into the crystal lattice of the metallic oxide by calcination at sufficiently high temperatures. The metal dopant incorporation into the crystal lattice of the metallic oxide is verified by X-ray diffraction. This catalyst composition shows high activities towards alkane conversion but suffers from the disadvantage of a low selectivity towards alkane isomerization and a high cracking selectivity towards gaseous C1-C4 products with low commercial value.
The preparation of mechanically stable sulphated zirconia catalysts by addition of alumina is described in patent literature for example in U.S. Pat. No. 6,326,328.
When alumina is present in tungstated zirconia catalysts it may be incorporated in the zirconia crystal lattice as aluminium ion (Al3+). Conventional catalysts in this form have been found to have a low selectivity to the isomerization products and tend to crack the multi-branched hydrocarbons produced to undesirable gaseous products.
The general objective of the invention is to provide a process for the preparation of a catalyst which is suitable for improving the octane number of a C4+ hydrocarbon mixture through isomerization without substantial cracking of the produced multi-branched hydrocarbons to gaseous products.
It is another objective of the invention to provide a catalyst, which is suitable for improving the octane number of a C4+ hydrocarbon mixture through isomerization without substantial cracking of the produced multibranched hydrocarbons to gaseous products.
It is a further objective of the invention to provide a catalyst, which contains reduced amounts of alumina while showing improved isomerization properties and reduced cracking properties.